This invention relates to a method for monitoring a Mass Flow Controller (MFC) connected to a pressure chamber for supplying gas to the chamber.
Mass Flow Controllers are used to supply measured flows of gas to pressure chambers for a range of processes including etching semi-conductor substrates and depositing films thereon. Accurate flow rates can be extremely significant in the reproducibility of the process and also inaccuracies of the flow rate could result in a different process being performed due to the change in proportion of chemicals within the chamber.
Systems have therefore been developed already for monitoring Mass Flow Controllers to identify problems in their operation. Typically these comprise arranging the desired flow rate at the MFC, allowing stabilising gas flow, allowing the fixed volume chamber to fill with gas for a fixed time or until a set pressure is achieved and determining the rate of fill of the gas into the chamber. During this test the chamber is not pumped. This rate value can be compared with a known good value to verify whether the MFC is operating correctly.
U.S. Pat. Nos. 7,822,570, 6,955,072 and 5,684,245 indicate various approaches to seeking to operate mass flow controllers.
Such a system is very practical for the historical relatively long process times. However, when using the well known switched Bosch process it is desirable to reduce the cyclic process time with the result that if the opening or closing times of the MFC vary from their design length, that deviation can become a significant part of the cycle time, causing distortions in the process for the reasons set out above. Put another way if there is a few milliseconds variation in the period of supply of gas to the chamber for a process or cycle which is going to last for many minutes, this leads to a negligible variation. If, however, the process or cycle time is significantly reduced, the lead or lag period in the operation of the MFC becomes a significant portion of the whole leading to process variations.
The Applicant has developed methods which can mitigate against this problem.